With a conventional method and apparatus for measuring blood pressure, it is difficult to directly measure the pressure of blood pumped from the heart. Thus, methods of indirectly measuring blood pressure have been used. A Korotkoff method is a representative method among these methods. In the Korotkoff method, the humeral region of a patient is wound with a band, the band is expanded by injecting air into the band until blood does not flow through blood vessels, and the blood pressure of the patient is measured using sound generated by gradually reducing pressure applied to the band. That is, when pressure is applied to the band by injecting air into the band and the amount of the air is gradually reduced, the sound of the pulse of the artery is heard similar to the sound heard when an object is feebly tapped. In this case, the pressure at this moment is systolic pressure which is blood pressure when the heart contracts. Then, when the amount of the air injected into the band is further reduced, there is a moment that the sound of the pulse increases more and then suddenly vanishes. The blood pressure at this moment is diastolic pressure, i.e., pressure in the blood vessels when the ventricles of the heart contract and expand again.
As another example, the SPO2 method has been also used to measure blood pressure. In the SPO2 method, oxygen saturation in a capillary vessel is measured and the blood pressure of a subject is determined based on the level of hemoglobin in blood flow.